Even though Real Madrid have already got Joselu, he was only on loan for one season from Espanyol and next year requires Real Madrid to hunt for another striker if this summer they don't get a striker. Mbappe and Osimhen are the right choices but until now there has been no clear offer for these two players.
Mbappe and Osimhen are two top-tier forwards Real Madrid may sign right now to replace Benzema. The void must be filled before the start of the next season, or Real Madrid will be without a competent striker to fill that position, which is critical for any club. The transfer window is still open, so we can't conclude that Real Madrid won't be able to sign any of them; it might be a last-minute decision.
Since Mbappe refused to extend his contract with PSG and since the club president Al Khalifi said it clearly that Mbappe has to renew the contract or leave the club in two weeks then I assume Mbappe will join Real Madrid by the end of the transfer window. At worst he will be sanctioned and will be in the benches the next season so he might accept to leave in the winter transfer window.
Real Madrid has to find a temporary solution only concerning that spot.
If they are unable to sign their preferred striker by the end of the transfer window, I believe Joselu should be the backup. Joselu may not be as good as Mbappe, but he can fill in for the time being until they can sign their first-choice striker. Joselu's striking strength could also increase if he realises there will be a competition for his role to be replaced by another striker.